Boat Thru-Hull Fittings, Vents and Drains: Matching the Hose

Quick Answer

Match the hose first. The size in a thru-hull table is the hose bore the fitting is made to take, so a 3/4 inch fitting goes with 3/4 inch hose, and the outside diameter of the body is a separate figure in the same table. Published stainless thru-hulls run from 1/2 inch to 1-1/2 inch, nylon outlets with a check valve from 5/8 inch to 2 inch, and flange drains from 1 inch to 1-1/2 inch. After the hose, decide the job: an outlet discharges water, a drain lets it go and stops it coming back, and a vent moves air. Then confirm the hole and what closes the fitting inside the hull.

Real-World Explanation

A thru-hull is a small part with an unusually large number of interfaces: a hose at one end, a hull at the other, a hole through structure, a valve or seacock inside, and a direction of flow that may only be allowed one way. Buying it by the picture is how boats end up with a fitting that needs a hose size nobody stocks, or a vent installed where a drain was meant to go.

Start with the hose, because hose is the part you cannot easily change. The published stainless thru-hull series runs 1/2, 3/4, 1, 1-1/4 and 1-1/2 inch, with body diameters of 14, 18, 24.5, 32 and 38 mm and overall heights between about 87 and 97 mm. The nylon outlet with a check valve goes further, from 5/8 inch to 2 inch, with a table that separates the nominal size from five lettered body dimensions. In both cases the nominal size is the hose bore, and the millimetre column is the metal. Two fittings with the same nominal size from different families are not interchangeable: a 1 inch nylon outlet and a 1 inch stainless thru-hull are different bodies with different hole requirements.

The Size Column Is a Hose Bore, Not a Hole Saw Size

The most expensive version of this mistake is drilling the hull to a size taken from the fitting’s nominal figure. A 1 inch thru-hull does not need a 1 inch hole; the hole has to suit the body and the shoulder that seals against the hull, and the hose has to suit the tail. Confirm the hole size, the body diameter and the length through the hull from the drawing, and check the hull thickness at that point. A fitting whose threaded body is shorter than the hull plus its inner skin will not seal properly however carefully it is bedded.

On a cored hull, the hole exposes the core, and the core needs sealing before the fitting goes in; that step is part of the installation rather than part of the product, but it is the reason a hull fitting job is rarely as quick as it looks. The published flange drain illustrates the same point from the other side: it lists a flange outline and a body depth, and both have to fit the panel it passes through.

Vent, Drain and Discharge Are Different Jobs

A tank vent moves air. The published 90-degree tank vent is a 316 fitting with a 45 mm overall diameter, a 22 mm thread and an 88 mm overall height, mirror polished, intended for fuel tanks and ventilation. It is not a drain and it is not a discharge, and installing one where water is meant to leave will not work however good the fit is. A vent also has to be positioned so that it breathes rather than filling with water: a vent that sits where spray lands constantly will let water into the tank, which is a worse problem than a slow drain.

A drain lets water out and, in some designs, keeps it from coming back. The published cockpit drain scupper is a 38 mm unit with a ball valve described as acting as a one-way valve, so water leaves the cockpit and is restricted from returning. That one-way behaviour is useful on a low cockpit drain and unhelpful if it is fitted the wrong way round or if the ball is allowed to foul. A check valve on an outlet does a similar job in a different housing: the published nylon outlet combines a nylon body with a stainless top cover and a check valve, and its table covers seven hose sizes from 5/8 inch to 2 inch.

Below the Waterline Is a Different Job

A thru-hull that sits below the waterline is not a standalone part; it is one piece of an assembly that includes a valve at the inner end, a hose and two clamps per joint. Confirm with the yard how the fitting is to be closed inside the hull and where the valve sits, because the through-hull itself does not close. The same applies to hose runs: a hose that dips below the waterline before it rises to its outlet can hold water, and a hose that is too long or kinked at the tail will fail at the connection rather than in the middle. Buy the hose in the bore the fitting states, cut it with a clean square end, and use two clamps at each end of a below-waterline run.

Above the waterline the rules are less severe but the consequences of a bad fit are the same in a following sea. A fitting that is bedded on a surface that is not flat, or that is tightened until the flange distorts, will leak where the seal is thinnest.

Practical Insights from Marine Hardware Supply

1. Send a cut length of hose with the enquiry

Hose bore varies with wall thickness and with the maker, and a 1 inch hose from one supplier can be a tight fit on a fitting from another. A short sample removes the argument.

2. Ask which direction the check valve works in

The published check valve outlet and the ball-valve scupper both restrict reverse flow, but that is only useful if the valve is fitted the right way round. Confirm the intended flow direction at the quotation stage, not after the hull is drilled.

3. Treat the lettered dimension columns as drawing-only data

Several of these tables use A, B, C and so on rather than named dimensions. Those figures are meaningful with the drawing that defines them, so request it for the exact model and check the flange, the body depth and the hole against your boat.

Buying Checklist

- Hose bore: the size of the hose you will use, measured, not assumed from the fitting label.

- Fitting family: outlet, drain, scupper or vent, chosen for the job rather than for appearance.

- Body diameter and length: how much of the fitting passes through the hull, against the hull thickness at that point.

- Hole size: taken from the drawing, not from the nominal size in the table.

- Flange and footprint: the outline that has to sit flat on the surface, and the fastening positions.

- Flow direction: whether a check valve or ball valve is wanted, and which way it should pass water.

- Closure inside the hull: how the fitting is closed, and where the valve sits.

- Hose run: length, route and whether the run dips below the waterline before it rises.

- Fastening and bedding: clamps for the hose, sealant for the flange, and fasteners for a flanged body.

- Records: part code, size and drawing revision for a repeat order.

Decision Guide

Choose a Straight Thru-Hull If

- A hose has to leave or enter the hull at a single point.

- The hole, the body length and the hull thickness have been checked against the drawing.

Choose an Outlet With a Check Valve If

- Water should leave easily and be restricted from returning.

- The valve direction suits the run and can be serviced or replaced in place.

Choose a Flange Drain If

- A cockpit sole or locker needs a flat flange to sit against, with a hose below.

- The flange outline fits the panel and the body depth clears what is underneath.

Choose a Cockpit Scupper If

- The drain is in a cockpit where water can wash back, and one-way behaviour helps.

- There is room for the valve body and access to clear it.

Choose a Tank Vent If

- A tank or a closed space needs air, and the position is clear of constant spray.

- The thread or spigot size matches the hose or fitting it connects to.

Defer Ordering If

- The hose bore or the hull thickness at the fitting point has not been measured.

- The closure inside the hull, or the flow direction of a valve, is still undecided.

The rule that matters most: match the hose bore first, then confirm the hole and the body length from the drawing, because the fitting is the cheap part of getting it wrong.

Need Help Choosing or Sourcing?

Send the hose bore, the hull thickness at the fitting point, the job the fitting has to do and the quantities you need, and contact Alastin Marine for a match against the thru-hull, vent and drain range, a sample or bulk supply. The published options include the 316 thru-hull with hose connection from 1/2 to 1-1/2 inch, the nylon thru-hull outlet with check valve from 5/8 to 2 inch, the flange drain with stainless top cover in 1, 1-1/8 and 1-1/2 inch, the 38 mm cockpit drain scupper with ball valve and the 90-degree 316 tank vent. For a fitting made to a specific hose bore, thread or flange outline, send the drawing through the OEM/ODM enquiry so the quotation starts from the documented interface.